Weak normal map bake?
When looking straight at a model, there must be visible curvature on the model for the normal map effect to be visible.

I've illustrated this using several cubes and a 3D Viewport Matcap Normals' surface.  The view is top-down orthographic.

The cube at the top-left is just a default cube with no bevel.  It is invisible to normal mapping.

The top-right cube has only a very small bevel and it is barely visible in contrast to the plane.

The two bottom cubes were beveled enough so that the curvature across the top and side surfaces is clearly visible.